Infrared (IR) detectors are often utilized to detect fires, overheating machinery, planes, vehicles, people, and any other objects that emit thermal radiation. Infrared detectors are unaffected by ambient visible light conditions and often even particulate matter in the air such as smoke or fog. Thus, infrared detectors have potential use in night vision and when poor vision conditions exist, such as when normal vision is obscured by smoke or fog. IR detectors are also used in non-imaging applications such as radiometers, gas detectors, and other IR sensors.
Infrared detectors generally operate by detecting the differences in thermal radiance of various objects in a scene. That difference is converted into an electrical signal which is then processed. Microbolometers are infrared radiation detector elements that are fabricated on a substrate material largely using traditional integrated circuit fabrication techniques. Microbolometer detector arrays consist of thin, low thermal mass, thermally isolated, temperature-dependent resistive membrane structures. They are typically suspended over silicon readout integrated circuit (ROIC) wafers by long thermal isolation legs in a resonant absorbing quarter-wave cavity design.
Conventional infrared detector arrays and imagers operating at ambient temperature include microbolometer arrays made of thin films of hydrogenated amorphous silicon (a-Si:H) or vanadium oxide (VOx). Other materials used for microbolometer arrays include films of various metal (e.g., titanium) and high temperature superconductors. For an array based on amorphous silicon, the detector pixel membrane is generally comprised of an ultra-thin (˜2000 Å) a-SixNy/a-Si:H/a-SixNy structure. The membrane is deposited at a low temperature nominally below 400° C. using silane (SiH4) and ammonia (NH3) precursors for the amorphous silicon nitride (a-SixNy) layers, and using silane for the hydrogenated amorphous silicon (a-Si:H) layer. Hydrogen atoms from silane (SiH4) molecules are the source of hydrogen content in the a-Si:H layer. A thin absorbing metal layer such as Titanium (Ti), Titanium-Aluminum alloy (TiAl), Nichrome (NiCr), black gold, or other material absorbing in the infrared band of interest, (e.g., at wavelength range of 1 micron to 14 micron), is inserted in the membrane to enhance infrared absorptance. Contact between the a-Si:H detector electrodes and the interconnect pads on a complementary metal oxide semiconductor (CMOS) signal processor of the ROIC is accomplished by metal interconnects. Precursors and alloy compositions of a microbolometer membrane structure may be adjusted by doping to improve stability and performance as described in U.S. Pat. No. 7,718,965.
After fabrication, microbolometers are generally placed in vacuum packages to provide an optimal environment for the sensing device. Conventional microbolometers measure the change in resistance of a detector element after the microbolometer is exposed to thermal radiation. Microbolometers have applications in gas detectors, night vision, and many other situations.